Silicon carbide (SiC) has a combination of electrical and physical properties that make it attractive for a semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 electron-volt (eV) bandgap, a 4 millivolt per centimeter (MV/cm) electric field breakdown, a 4.9 W/cm.multidot.K thermal conductivity, and a 2.0.times.10.sup.7 centimeter per second (cm/s) electron drift velocity. Furthermore, because SiC will grow thermal oxides, it offers significant advantages over other compound semiconductors. In particular, the ability to form a thermal oxide raises the corresponding ability to form metal-oxide-semiconductor (MOS) devices, including MOS field-effect transistors (MOSFETS) insulated gate bipolar transistors (IGBT's), MOS-controlled thyristors (MCTs), and other related devices. In turn, MOSFETs are extremely important devices in large scale integrated circuits. Accordingly, taking full advantage of SiC's electronic properties in MOS devices and resulting integrated circuits requires appropriate SiC oxidation technology.
In spite of these advantageous characteristics, the quality of oxides on silicon carbide (SiC) has been a major obstacle to developing silicon carbide MOS devices. Oxide charge and interface states in particular, have an adverse effect on MOS device performance. Indeed, with the exception of substrate crystal growth, oxide quality is perhaps the largest barrier to advanced SiC MOS power devices and MOS integrated circuits. Oxides on SiC have been widely reported to have unacceptably high interface state densities and fixed oxide charge for MOS applications.
As used herein, the term "state" refers to a characteristic of a material, such as an available energy level position, and often defined using quantum mechanics. The existence of "states" in materials is not necessarily good or bad, and their presence or absence is probably better defined as desired or undesired. In the case of oxide layers on silicon carbide, the states appear to produce undesired operating characteristics in resulting devices, and thus are desirably minimized whenever possible.
Previous work comparing thermal oxidation techniques and investigating SiC/SiO.sub.2 defects have led to only a few general observations: (1) wet oxides (i.e. those thermally grown in the presence of water vapor) have lower oxide charge and interface state densities than dry oxides (S. M. Tang, W. B. Berry, R. Kwor, M. V. Zeller and L. G. Matus, J. Electrochem. Soc., 137(1) p.221 (1990); S. Zaima, K. Onoda, Y. Koide, and Y. Yasuda, J. Appl. Phys., 68(12) p. 6304 (1990)); (2) the Si-face of SiC wafers oxidizes more slowly, but has lower oxide charge densities, than the carbon (C) face (Nitya N. Singh and Andrew Rys, Inst. Phys. Conf. Ser. No 137: Chapter 3, p. 325 (1993)); (3) oxides grown on n-type wafers have fewer interface states (Dale M. Brown, Mario Ghezzo, James Kretchmer, Evan Downey, Joseph Pimbley, and John Palmour, IEEE Trans on Electron Devices, Vol. 41, No. 4, p. 618 (1994)); T. Ouisse, N. Becourt, F. Templier, C. Jaussaud, J. Appl. Phys. 75(1) (1994); Singh, supra) and lower oxide charge densities (Brown, supra; M. Shinohara, M. Yamanoka, S. Misawa, H. Okumura, S. Yoshida, Jap.J.Appl.Phys. 30(2) p.240 (1991); Singh, supra) than those grown on p-type wafers; and (4) contrary to early projections (Brown, supra), there is no difference between boron (B) and aluminum (Al) as a p-type dopant in SiC (J. N. Shenoy, L. A. Lipkin, G. L. Chindalore, J. Pan, J. A. Cooper, Jr., J. W. Palmour, and M. R. Melloch, Inst. Phys. Conf. Ser. No 141: Chapter 4, p. 449 (1994)).
It will be understood that for the most part, the accuracy of these measurements and the resulting conclusions can vary to a greater or lesser extent, and should be evaluated and understood in that light.
Although these particular conclusions are widely accepted, the quantification of the net oxide charge (Q.sub.ox) and interface state densities (D.sub.it) has varied widely, even when measuring the same crystal orientation, oxide thickness, dopant type and dopant species. These variations are demonstrated in Table 1, which summarizes the oxide charge and interface state densities of SiO.sub.2 on SiC as reported by various authors. The reported oxide thicknesses range from 500-600 .ANG., except for the 200 .ANG. oxides on n-type samples of reference reported by Weiss, supra.
TABLE 1 ______________________________________ Substrate/ Q.sub.ox D.sub.it Oxide (cm.sup.-2) (cm.sup.-2 eV.sup.-1) ______________________________________ N-6H/dry 0-1 .times. 10.sup.11 0.5-5 .times. 10.sup.12 N-6H/wet negligible 0-2 .times. 10.sup.11 p-6H/wet mid 10.sup.12 0.15-5 .times. 10.sup.12 ______________________________________
These seemingly inconsistent results probably reflect the variety of measurement techniques used. As Table 1 demonstrates, the largest variations are in the reported interface state densities. The net oxide charge is straightforward to measure with a standard capacitance-voltage (CV) sweep at room temperature. Accurately calculating the interface state density, however, can be quite challenging. Raynoud et al. (Christophe Raynoud, Jean-Lun Autran, Bernard Balland, Gerard Guillot, Claude Jaussaud and Thierry Billon, J. Appl. Phys. 76 (2) p. 993 (1994)); Shenoy, supra; and Ouisse, supra; have discussed the various methods that can be used and the assumptions and limitations that must be taken into account when determining interface state densities. Of all the different techniques, the conductance and high-low techniques seem to be the most consistent and repeatable.
Although the reported quantities of oxide defects vary significantly from group to group, the goals for high quality MOS applications are fairly well defined: Oxide charge less than 1.times.10.sup.11 cm.sup.-2 and interface states less than 5.times.10.sup.10 cm.sup.-2 eV.sup.-1. As Table 1 demonstrates, the quality of thermal oxides on SiC needs dramatic improvement. There are, however, few reported techniques for improving oxide quality. To date, the largest improvements result from varying the insert/withdrawal conditions of the oxidation and the oxidation pre-clean. Shenoy et al. (J. N. Shenoy, G. L. Chindalore, M. R. Melloch, J. A. Cooper, Jr., J. W. Palmour, and K. G. Irvine Inst. Phys. Conf. Ser. No 141: Chapter 4, p. 449 (1994)) also report a minimum net oxide charge density of 9.times.10 cm.sup.-2, and a interface state density of 1.5.times.10.sup.11 cm.sup.-2 eV.sup.-1, which are 3-4 times lower than their earlier oxidation conditions.